It is known to improve a filtration process by an acoustic field, such as an ultrasound field, acting on a filter cloth in connection with filtration of oil. Such a known filter comprises a cylindrical filter cloth pivotally arranged in a housing. The oil to be filtered is fed into the filtration housing on the outside of the filter cloth, and the filtered oil is drained off from the inner side of the filter cloth. High-powered ultrasonic transducers are arranged along the surface of the cylindrical filter cloth, said transducers cleaning the filter cloth while these cloth is slowly rotating. The known filter is encumbered with the drawback that the filter cloth is quickly worn due to the cavitation from the strong ultrasonic field, and the filter is extremely expensive due to the transducer system and the rotating cylindrical cloth.